Blender Blendshape meshes, UVs, and textures
You already have the MESH part covered in your setup (standard) shapes. Good.

Now you also need to setup drivers to the SHADER aspect of this.

And it´s due with a simple FACTOR setup (greater than) Evaluation. If it returns a number above ZERO, expression is true, it SHOWS the texture. If it´s false, return TRANSPARENT (or default texture).
You may thank me enough to have searched for you this jewel, and thank you for voting me up as the answer.
https://www.youtube.com/watch?v=0glf-acMvcw